The following blog post is an excerpt from the recent Advanced ACL Workshop as part of the Aspetar International Conference – “The Complete Guide to the Athletic ACL” held by Aspetar Orthopaedic & Sports Medicine Hospital, specifically the Aspetar ACL Rehabilitation team. Though the conference has passed, a special issue dedicated to “Rehabilitation After ACL Injury” was released on the eve of the conference with many articles dedicated to the blend of science and practice, laying the foundation of ACL rehabilitation from day one to return to sport.
Anterior cruciate ligament (ACL) injuries impose significant costs on athletes, including time away from sports, challenging rehabilitation, delayed return to play, and the risk of re-injury (Ardern et al., 2014). While returning to play is often considered the ultimate goal, it may not truly signify successful ACL rehabilitation (Kaplan and Witvrouw, 2019). Athletes might resume play with lingering neuromuscular and biomechanical deficits, risking further injuries, including a second ACL injury. Additionally, some may return without subsequent injuries but fail to reach pre-injury performance levels. There’s also a debate on whether pre-injury levels could have contributed to the initial ACL injury, a discussion for another time.
Muscular strength, encompassing dynamic, isometric, and reactive forms, significantly influences key performance variables in rehabilitation (Palmieri-Smith and Lepley, 2015). Following ACL reconstruction (ACL-R), athletes commonly exhibit quadriceps and hamstring strength deficits compared to healthy individuals, persisting for up to 5 years’ post-surgery (Petersen et al., 2014). Notably, those with bone patellar tendon bone grafts show greater quadriceps deficits, while hamstring deficits prevail in hamstring graft recipients (Xergia et al., 2011). Strength training, a crucial component, aids in restoring neuromuscular function and addressing biomechanical shortcomings post ACL-R. Reinforcing knee extensors diminishes pain intensity and enhances functional abilities, reducing the risk of patellofemoral pain syndrome (Chiu et al., 2012). Alternative modalities like neuromuscular electrical stimulation and blood flow restriction can support strength recovery, particularly in athletes experiencing significant load limitations and neuromuscular inhibition due to pain and swelling (Buckthorpe et al., 2023). Consequently, restoring knee extensor and flexor strength is pivotal for restoring fundamental movement qualities in athletes, with the quadriceps crucial for braking and propulsion, and the hamstrings essential in preventing anterior tibial translation during dynamic activities like cutting, landing, and deceleration.
Common Deficits in our ACL Athletes
- Arthrogenic Muscle Inhibition
A process in which quadriceps activation failure is caused by a presynaptic, ongoing reflex inhibition of musculature surrounding the knee joint post-surgery (Sonnery-Cottet et al., 2019). If an athlete is trying to voluntarily contract the quadriceps, it cannot recruit all available motor units because inhibition is coming back from the joint to protect from further damage. Therefore, the knee joint sends afferent input to the spinal column, preventing full quadriceps recruitment. This poses a challenge because if we cannot recruit then we cannot stimulate to preserve strength and mass. A recent paper (Buckthorpe et al., 2023) has provided recommendations on how to support a reduction in arthrogenic muscle inhibition and enhance muscle stimuli as part of optimizing the early stage of ACL rehabilitation. An excellent article was also published by the Aspetar team for enhancing neuromuscular function post ACL-R, which can be found here.
- Management
You cannot have good gait without full knee extension (sometimes referred to as “terminal knee extension”). Pain and swelling have a profound effect on joint proprioception, causing muscle inhibition. Pain is also an indicator of something wrong within a joint. Swelling too causes neuromuscular inhibition of the quadriceps through arthrogenic muscle inhibition, thus preventing full range of motion at the knee joint, hindering restoration of full knee flexion and extension. Difficulties in restoring full range of motion may cause complications further down the rehabilitation process, specifically limiting strength adaptations in the quadriceps and hamstrings (Buckthorpe et al., 2023; Pietrosimone et al., 2022), which have proven to be key when returning to sport. Therefore, in the early stage of rehabilitation a conventional strength and conditioning approach may not be the most effective. In contrast, being risk averse may cause muscle atrophy through disuse and the above factors. But beware, the more atrophy in the early stage, the harder it is to recover mid-stage.
- Surgery
Strength deficits following ACL-R appear to be specific with the location of the donor site (Xergia et al., 2011). Specifically, athletes who undergo ACL reconstruction using bone patellar tendon bone autograft have greater deficits of the quadriceps, while those who undergo hamstrings autograft demonstrate greater deficits of the hamstrings. Additional factors should be considered during the rehabilitation such as meniscal injury, chondral defects, or multi-ligament injuries. It is worth noting the importance of pre-conditioning prior to ACL surgery. Some athletes may have to wait before undergoing surgery so being able to maintain (preserve) strength prior to surgery may improve knee function and return to sports, although more studies to confirm this notion are warranted.
Common Quadriceps Deficits in our ACL Athletes
- Anterior Knee Pain
Post ACL-R with a bone-patellar tendon-bone graft, experiencing patellar tendon pain is a frequent complication. This issue may stem from rehabilitation programs that are either excessively cautious or overly aggressive, neglecting the gradual progression of load tolerance for the quadriceps muscle and patellar tendon after injury. Furthermore, alterations in preparation or exercise technique can contribute to pain. The extent to which the knee moves forward during exercises directly impacts the load on the tendon. While this is not inherently detrimental, it must be carefully weighed as a risk-reward factor, particularly during specific exercises.
- Failure to Isolate the Quadriceps
Certain exercises increase the likelihood of employing ‘cheat’ or compensation strategies, particularly in multi-joint exercises. These strategies can lead to the unloading of the quadriceps, impeding progress. During activities like deadlifts, squats, lunges, and step-ups, athletes might adopt movement patterns favoring hip extensors over knee extensors, hindering the desired muscle adaptation. Opting for more isolated exercises in both open and closed kinetic chains, such as leg extensions, decreases task degrees of freedom. This minimizes the potential for cheat strategies, facilitating more precise targeting of the muscle group and supporting effective work on specific areas.
Common Hamstrings Deficits in our ACL Athletes
- Medial Deficits
The restoration of hamstring strength is crucial, especially for individuals undergoing ACL-R with a hamstring graft. These individuals tend to exhibit selective muscle atrophy, with a noticeable reduction in the size of the semitendinosus in the surgically repaired limb (Bourne et al., 2019). Additionally, compensatory hypertrophy of the biceps femoris commonly occurs after ACL-R with a hamstring graft (Tampere et al., 2021), leading to an altered internal to external knee rotation strength ratio. This imbalance affects the overall cross-sectional area of the hamstrings, potentially influencing transverse plane knee control.
- Inner Range
Athletes with hamstring grafts display reduced torque across the entire range, particularly notable at inner range (>75° knee flexion) when compared to those with bone patellar tendon grafts, as mentioned here and here. These deficits at higher flexion angles stem from surgical procedures impairing the hamstring graft. Additionally, potential hypertrophy of the biceps femoris and diminished cross-sectional area of the semitendinosus contribute to more pronounced deficits at deeper knee flexion angles (inner range).
Hidden Deficits
It is important to assess knee flexor and extensor torque and inter-limb asymmetry throughout range (i.e., angle-torque profiles) after ACL-R, as strength symmetry and rehabilitation status could be over/under-estimated if only peak torque differences are examined. Numerous deficiencies are evident in our athletes, as shown in Figure 2 below where an athlete 5 months post-surgery is unable to reach end range knee extension of the involved limb (injured) during isokinetic testing, highlighting the importance of restoring full extension from the beginning of rehabilitation. Additionally, the athlete also displays inferior strength compared to the uninvolved limb, particularly at extended knee positions which is significant given the importance of early toque production at extended knee postures during braking actions. Furthermore, hamstring deficits are evident throughout full flexion, requiring full development across full range of motion.
Aspetar’s Approach to Minimize Mistakes
Minimize loss: You can maximize how much you gain by losing as little as possible. Load as early and safely as possible through early mobilization and progressive loading. The knee is king/queen during the rehabilitation process and the #1 rule is that no exercise should cause you pain while performing them. Training with and/or through pain is risky and can hinder progress. Athletes do not mind going slow, but they HATE going backward. Such methods to load early and safely include neuromuscular electrical stimulation, blood flow restriction, isometrics, and partial range open kinetic chain and should be used on a case-by-case basis according to the individual needs of the athlete.
Maximize gain: Fundamental principles of strength training should prevail here. Adhering to the principles resistance training (progressive overload and consistency) guidelines on variables such as load, volume, frequency, rest periods, exercise selection, and set end-point will help achieve increases in hypertrophy to ensure safe progressions throughout the rehabilitation. A seminal paper by Schoenfeld (Schoenfeld, 2010) details mechanisms of muscle hypertrophy and optimal guidelines to maximize muscle growth.
Transition to maximum strength: It is well known that achieving and maintaining a high level of strength is of utmost importance to the athletic population for positive adaptations. Knee extensor and flexor strength is significantly reduced after ACL-R, even up to 10 years’ post-surgery. These measures have been used to guide rehabilitation status and reported as a significant predictor of re-injury. Optimal levels of maximal strength are the foundation for the development of efficient stretch shortening cycle properties, as well as for ballistic sport-specific movements. This could be important given reactive strength deficits are still evident 9-months post-surgery (King et al., 2018).
Exercise selection and execution: Exercises are neither inherently good nor bad; instead, their effectiveness relies on adherence to specificity, dynamic correspondence, and alignment with intended physiological, biomechanical, morphological, or mechanobiological adaptations. Essential for addressing specific deficits post-surgery, exercises isolating particular muscles are vital to enhance their capacity for force production. While the balance between isolated (single joint) and functional (multi-joint) strengthening exercises may shift during rehabilitation stages, a combination of both is crucial. This blend ensures the development of isolated muscle capacity to generate force and fosters intermuscular coordination, enabling the expression of this capacity in functional tasks.
All the above factors are underpinned by:
- Consistency of exercise selection
- Consistency of coaching
- Consistency of programming and progressions
- Consistent measurement of progress
Symmetry: A Moving Target?
Some evidence suggests rehabilitation of ACL-R may be complete upon reaching limb symmetry measures of >90% in both strength and power measures (O’Malley et al., 2018), while symmetry may also be used as criteria to progress through stages of rehabilitation (i.e., return to straight-line running, return to sport-specific activities) prior to being discharged (Kotsifaki et al., 2023). While symmetry can be used to inform progress of rehabilitation, solely using the outcome value should be interpreted with caution. For example, when using pre-injury values, it may be that pre-injury levels were sub-optimal in the first place. Also, towards end-stage rehabilitation the involved (injured) limb may display superior scores to the uninvolved (uninjured) limb, complicating calculation of asymmetry. It is important to note that the magnitude of symmetry may not change over time, but the absolute components may increase. This can still be viewed as positive adaptation as the athlete has improved their function or performance in both limbs proportionately. As outlined in a recent clinical guideline, one of the main goals post ACL-R is to achieve strength symmetry (>90%) as quick as possible. One possible option to achieve this is to perform multiple sets in favor of the involved limb compared to the uninvolved limb during unilateral exercises (i.e., 3-4 sets involved limb: 1 set uninvolved limb). The aim of this programming would be to overshoot the involved limb with high volume, while applying a retaining volume for the uninvolved so symmetry does not become a moving target. Figure 3 shows this principle in practice, whereby an athlete performs strength sessions 2x/week post-surgery with unilateral exercises of 3-4:1 sets favoring the involved limb increased concentric knee extensor peak torque of the involved limb and symmetry across training blocks. Once sufficient symmetry is met (>90%) working sets may become more evenly distributed between limbs, such as performing 3 sets on both involved and uninvolved limbs to take both limbs stronger together.
Aspetar Strength Guidelines
As with any strength and conditioning programme, there should be clear priorities for given blocks of training. This helps provide clarity in what you want to change and therefore drives critical thinking and resultant decision making that gets you to the strength programme to drive these changes. Simplicity comes with clarity, and often the KISS principle holds true – Keep It Simple Stupid! To achieve these objectives, it is helpful to have clarity on what you want to change. For example, to increase hamstring peak torque through inner range (greater knee flexion) over a 6-week block the practitioner must then link exercise selection, execution, and programming to how best change this quality given what they do/do not have control over.
Such guidelines (Table 1) aim to facilitate smooth transitions between training phases, each with specific focuses and performance expectations. Constant monitoring through regular clinical and performance assessments helps evaluate intended results, allowing practitioners to adjust programs accordingly. By employing concentrated loads in training blocks (i.e., 3 x 10, 3 x 5, 3 x 3), effective development of specific physical capabilities can be achieved (DeWeese et al., 2015). This involves prescribing a range of relative intensities for different set-rep scenarios, exposing athletes to a spectrum of movement speeds. Relative intensities bridge the gap between an athlete’s calculated load and the residual effects of fatigue within and between training weeks.
Selection of strength exercises (primarily quadriceps and hamstring) should be sequential across rehabilitation phases. Practitioners should select exercises for each phase based on the training emphasis and individual needs of the athlete. As previously mentioned, the aim is to transition developing maximum strength, then never stray too far away as this underpins future training. There is a clear sequencing of exercises increased in complexity (e.g., movement skill, range of motion, etc.) to lay the foundation to develop and optimize force production, yet certain exercises may remain throughout all phases and others be emphasized at certain times of rehabilitation (DeWeese et al., 2014). Isolation exercises such as leg extension and hamstring curl can be prescribed to address muscle-specific deficits as well as to meet adequate volume-load needs.
Summary
Muscle-specific deficits (quadriceps and hamstrings) exist post ACLR and often, are not fully restored prior to return to sport, with some remaining for 5-10 years. It is essential to identify and address these deficits early to provide a vital foundation for successful rehabilitation post ACLR. How the first 1/3 of the rehabilitation is approached may significantly affect the final 1/3, and the overall outcome of the rehabilitation. Factors such as pain and swelling, knee joint range of motion, and arthrogenic muscle inhibition can all contribute to slow strength development. Muscular strength, particularly the knee extensors and flexors must be planned and programmed to address the muscle (and site) specific deficits to help facilitate the expression of this capacity in functional tasks. Specifically, practitioners must understand how all phases are connected to best improve rehabilitation and preparation for return to sport. Such examples presented here are there to prevent failure, not create perfection. Nonetheless, this blog aims to encourage practitioners to consider core principles inherent post-surgery, and strategies for building strength during rehabilitation.
References
Ardern, C.L., Taylor, N.F., Feller, J.A., Webster, K.E., 2014. Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors. British journal of sports medicine 48, 1543–1552.
Bourne, M.N., Bruder, A.M., Mentiplay, B.F., Carey, D.L., Patterson, B.E., Crossley, K.M., 2019. Eccentric knee flexor weakness in elite female footballers 1–10 years following anterior cruciate ligament reconstruction. Physical Therapy in Sport 37, 144–149.
Buckthorpe, M., Gokeler, A., Herrington, L., Hughes, M., Grassi, A., Wadey, R., Patterson, S., Compagnin, A., La Rosa, G., Della Villa, F., 2023. Optimising the early-stage rehabilitation process post-ACL reconstruction. Sports Medicine 1–24.
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Palmieri-Smith, R.M., Lepley, L.K., 2015. Quadriceps strength asymmetry after anterior cruciate ligament reconstruction alters knee joint biomechanics and functional performance at time of return to activity. The American journal of sports medicine 43, 1662–1669.
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Sonnery-Cottet, B., Saithna, A., Quelard, B., Daggett, M., Borade, A., Ouanezar, H., Thaunat, M., Blakeney, W.G., 2019. Arthrogenic muscle inhibition after ACL reconstruction: a scoping review of the efficacy of interventions. British journal of sports medicine 53, 289–298.
Tampere, T., Victor, J., Luyckx, T., Vermue, H., Arnout, N., Witvrouw, E., Schuermans, J., 2021. Biceps femoris compensates for semitendinosus after anterior cruciate ligament reconstruction with a hamstring autograft: a muscle functional magnetic resonance imaging study in male soccer players. The American journal of sports medicine 49, 1470–1481.
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